1. Field of the Invention
The present invention relates to a method of controlling the output voltage of an induction generator.
2. Discussion of the Related Art
Direct Torque Control (DTC) was developed to simplify the control of induction motors and improve their dynamic performance. The DTC concept was first explained in two independent, almost simultaneous publications. The first publication was a paper by Takahashi et al., entitled xe2x80x9cA New Quick Response and High Efficiency Strategy of an Induction Motorxe2x80x9d, Conf. Record, IEEE-IAS 1985 Ann. Meeting, pp. 495-502. The second publication was in Germany by Depenbrock entitled xe2x80x9cDirect Self Control for High Dynamic Performance of Inverter Fed AC Machinesxe2x80x9d, ETZ Archiv, Vol. 7, No. 7, 1985, pp. 211-218. Subsequently, various variations of this concept, always applied to induction motors, were presented.
One example of an alternate method of DTC application to induction motors is given in the paper by Lascu et al., entitled xe2x80x9cA Modified Direct Torque Control (DTC) for Induction Motor Sensorless Drivexe2x80x9d, IEEE-IAS 1998 Ann. Meeting, pp. 415-422.
The present invention seeks to provide a method of controlling an induction generator that is simpler and less expensive than previous methods.
The invention is primarily concerned with controlling an induction generator with a phase number equal to or greater than 3.
Most of the proven benefits of DTC concept can be realized if the method is modified so that it can be used to control an induction generator. This is especially true in cases where the generator shaft torque (also known as the loading torque on a prime mover) needs to be controlled. These types of situations are encountered in automotive applications, where the retarding torque that the generator exerts on the crankshaft of an Internal Combustion Engine (ICE) needs to be controlled. This situation is also encountered in windmills, where the retarding torque of the generator needs to be regulated to prevent a windmill from stalling.
This application discloses a modified method of Direct Torque Control that is applied to induction generators.
The present invention proposes the use of a minimal number of current sensors and the elimination of current regulators while controlling the generator flux and output voltage.
One method of controlling the induction generator according to the present invention is by using DTC. DTC differs from vector control in that vector control requires current regulators, while DTC does not. In its original form, DTC only required regulation of torque and flux. In this application, the invention discloses how to regulate the machine flux and either the torque or the generator output voltage.
The present invention deals with control of induction generators and is inspired by DTC concepts, previously applied only to motor control. The features that distinguish this invention from the prior systems include:
1. DTC is used to control induction generators;
2. Both generator torque and generator output voltage are controlled; and
3. No current regulators are used when defining the generator input voltages (Vd and Vq).
One application of the present invention is to an induction generator for automotive use and specifically to an induction machine automotive starter-alternator. Another application of the method of the invention is with a windmill.
The invention is also applicable to an induction machine with an electronically selectable number of poles.
An object of the present invention is to realize a minimal sensor implementation of a wide constant power speed range of a toroidally wound induction machine starter alternator (S/A), and specifically for generator mode voltage regulation.
Another object of the present invention is to provide a control method that is applicable to a system where an inverter is used to control a generator where the speed is variable and is not controlled. In the case of an automotive application, the speed is dependent on the speed of the automotive engine and thus is not controlled. In the case of a windmill, the speed is dependent on the wind speed passing by the blades of the windmill that is also not controlled.
Yet another objective of this invention is to control the generator operating point and specifically the loading torque the generator exerts on the prime mover, such as an internal combustion engine or a windmill.
These and other objects of the invention can be accomplished by various methods of controlling an induction generator, as will be described. The objects of the invention can be accomplished by a method of controlling an induction generator using a minimal number of current sensors and without requiring current regulators or position sensors.
This method comprises the steps of measuring a plurality of current amounts in the generator; transforming the plurality of current amounts into a two phase reference system; measuring a DC voltage supplied to an inverter; measuring a plurality of voltage amounts in the generator using voltage sensors; transforming the plurality of voltage amounts into the two phase reference system; calculating a flux in the generator using the currents and the voltages obtained by said steps of transforming so as to obtain a magnitude and position of the flux; comparing the calculated flux magnitude with a desired flux to determine a flux error amount; determining a d-axis voltage so as to reduce the flux error amount; determining a desired torque amount by obtaining a desired generator shaft torque amount and converting the desired generator shaft torque amount to the desired torque amount; comparing the desired torque amount with an estimated torque amount to determine a torque error amount, the torque error amount being input to a torque regulator; determining a q-axis voltage so as to reduce a torque error amount; and transforming the d-axis voltage and the q-axis voltage to stationary reference frame n-phase voltages using the position of the flux, wherein n is substantially equal to a number of generator phases.
The objects of the invention can also be accomplished by a method of controlling an induction generator using current sensors. The method of this invention includes the steps of measuring a plurality of current amounts in the generator using a plurality of current sensors; transforming the plurality of current amounts into a two phase reference system; measuring a DC voltage supplied to an inverter, the inverter being operatively connected to the generator; measuring a plurality of voltage amounts in the generator using a plurality of voltage sensors; transforming the plurality of voltage amounts into the two phase reference system; calculating a flux in the generator using the currents and the voltages obtained by said steps of transforming so as to obtain a magnitude and position of the flux; comparing the calculated flux magnitude with a desired flux to determine a flux error amount, the flux error amount being input to a flux regulator; determining a d-axis voltage so as to reduce the flux error amount; comparing a desired DC voltage with the measured DC voltage to determine a voltage error amount, the voltage error amount being input to a voltage regulator; determining a desired torque amount so as to reduce the voltage error amount; comparing the desired torque amount with an estimated torque amount to determine a torque error amount, the torque error amount being input to a torque regulator; determining a q-axis voltage so as to reduce a torque error amount; and transforming the d-axis voltage and the q-axis voltage to stationary reference frame, n-phase voltages using the position of the flux, wherein n is substantially equal to a number of generator phases.
It is also possible to use the magnitude and position of the rotor flux instead of the magnitude and position of stator flux as rotor flux magnitude and position can be calculated from the stator flux.